Present two-way FM radio communication systems, with closely spaced channels, place stringent requirements on their internal frequency determining elements. Some of which are stability, low frequency modulation response, and cost. To meet these requirements most receivers use a local oscillator to convert the incoming signal to a frequency at which it is easier to obtain selectivity and gain than at the carrier frequency. In the superhetrodyne method, the local oscillator frequency differs from that of the incoming signal by an amount equal to an intermediate frequency (IF); in the direct conversion method, the oscillator frequency equals the carrier frequency, which places the IF at baseband or zero frequency.
Closely spaced channels require receivers to have narrow band pass IF filters to reject adjacent channel interference and to obtain good sensitivity. Frequency tolerances in both the local oscillator and the received signal can allow the generated IF signal to fall outside the passband of the narrow filter. At UHF frequencies of 800 MHz and above, an error of only 2 parts-per-million (ppm) in the transmitted frequency could cause .+-.1.6 kHz error. The local oscillator could have similar tolerance and result in several kilohertz net frequency error at the IF. With the filter bandwidth required for 12.5 kHz channel spacing, errors of several kilohertz make it impractical to center the received signal in the IF filter passband without using additional frequency control means.
In tracking variations of the carrier frequency, local oscillator control loops interfere with demodulation of low-frequency FM information, which resides in instantaneous variations of the carrier frequency. Demodulation circuits expect these variations to be present after the carrier is translated to IF, but the control loops remove modulation components having frequencies within their loop bandwidths. The severity of this problem depends on the frequency content of the modulation signals and on the bandwidth of the control loop.
Typically, FM radio communication systems avoid modulation at DC, which requires shifts in the carrier frequency that are indistinguishable from static frequency errors. However, some signals, such as digital signalling data, may have frequency components at 10 Hertz or less. The transmission and reception of such data requires circuits capable of processing low frequency and even DC FM signals. With such wide spread use of data communications in today's systems, it is impractical for advanced communication systems to be incompatible with data transmission and reception.
In the design of synthesizers a means to achieve DC FM is by modulating the reference oscillator. Modulating the reference oscillator is costly and not desirable otherwise. Since the reference oscillator of the synthesizer is used by other circuits in the design such as filters, controllers, and micro-processors any modulation of the reference oscillator will have direct effect on the performance of mentioned circuits.
Alternatively, the conventional offset approach in the design of synthesizers requires a fairly high frequency offset VCO so that the undesired image response of the mixer output can be filtered effectively. This adds to the cost of the RF bandpass filter. Furthermore, it is impossible to achieve good center frequency accuracy of the offset oscillator without using a crystal, which would again add to the circuit cost.
Thus, a need clearly exists for an approach to produce DC FM without the high cost of any additional crystals or the requirement to DC frequency modulate the reference oscillator.